A device for transferring a fluid is known from DE 100 53 441 A1, which describes a device for supplying and/or branching off a secondary flow into or from a main flow of a medicinal fluid. The device here comprises a fluid-conveying body, in which a main channel which is open to one side and at least one secondary channel leading into the main channel are formed. The open side of the main channel is covered by a cover film, and the opening of the secondary channel into the main channel can be closed by the cover film. For closing the opening of the secondary channel, the cover film can be pressed by a valve actuator onto the opening of the secondary channel. This device can be used, for example, in an exchangeable cassette to be inserted into a fluid-conveying device such as an injection apparatus or a dialysis apparatus, for example. The valve actuators for closing the opening of the secondary channel are arranged here in the fluid-conveying device, and the cassette is inserted therein so that the valve activators come to be applied on opening sites where a secondary channel leads into the main channel, so that the openings of the secondary channels can be closed (alternately) due to an actuation of the valve actuators.
In the known transfer devices, one problem is, in the case of a negative pressure in the main channel or in a secondary channel, the cover film, which can be designed as a film or as a resilient membrane made from an elastomer material, can be pulled onto the opening of a secondary channel, even if the associated valve actuator is not actuated. Subsequently, it may happen that the secondary channel is closed unintentionally even if the valve actuator is not activated, due to a negative pressure in the main channel or the secondary channel. In a corresponding manner, when opening closed secondary channel openings, problems can also occur if a negative pressure develops in the area of the main channel or of the secondary channel. This can occur, for example, in the case of an actuated valve actuator and closed opening of a secondary channel, if the conveyance pump by means of which the fluid is conveyed through the channel still continues to run for a short time after the pump has been switched off. In the case of an unintentional opening of the sealing seat of the cover film on an opening of a secondary channel by actuating the valve actuator into its open position, it may occur that the cover film does not separate from its sealing seat on the opening surface of the secondary channel, due to the negative pressure in the secondary channel, and the secondary channel therefore remains unintentionally closed.